The invention relates to a single collimator lens having one aspherical surface. The invention also relates to an optical system producing a collimated radiation beam and to an optical scanning unit provided with such an optical system.
Netherlands Patent Application No. 8103323 (corresponding to U.S. Pat. No. 4,415,238) discloses a single lens having one aspherical surface (a monaspherical lens). This lens is suitable for use as an objective lens in an optical scanning unit for reading from or recording on an information surface of an optical record carrier.
The information surface must be scanned with a minute radiation spot whose diameter is on the order of 1 .mu.m. Consequently, the lens must have a comparatively large numerical aperture. Further, the objective lens must have a comparatively large diffraction-limited field in order to form a sharp radiation spot at points which are radially offset from the optical axis of the objective. This is necessary to correct for minor deviations in the position of the radiation spot relative to a track pattern in the information surface. The requirements of a comparatively large numerical aperture (N.A. larger than 0.25) and a large diffraction-limited field can be met by a monaspherical lens whose second surface is spherical. The first surface is highly aspherical and has a shape which is defined by a series expansion with a multitude of terms.